/*-------------------------------------------------------------------------
 *
 * pg_freespacemap.c
 *      display contents of a free space map
 *
 *      contrib/pg_freespacemap/pg_freespacemap.c
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "funcapi.h"
#include "storage/freespace.h"

PG_MODULE_MAGIC;

/*
 * Returns the amount of free space on a given page, according to the
 * free space map.
 */
PG_FUNCTION_INFO_V1(pg_freespace);

Datum
pg_freespace(PG_FUNCTION_ARGS)
{
    Oid            relid = PG_GETARG_OID(0);
    int64        blkno = PG_GETARG_INT64(1);
    int16        freespace;
    Relation    rel;

    rel = relation_open(relid, AccessShareLock);

    if (blkno < 0 || blkno > MaxBlockNumber)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("invalid block number")));

    freespace = GetRecordedFreeSpace(rel, blkno);

    relation_close(rel, AccessShareLock);
    PG_RETURN_INT16(freespace);
}
